Comparison of heat flux measurements by IR thermography and probes in the Alcator C-Mod divertor

Alcator C-Mod was recently upgraded with multiple diagnostics to measure the heat flux footprint on the outer divertor: IR thermography, plasma conditions from Langmuir probes (LPs), and embedded calorimeters to measure local energy deposition. In addition to providing consistency checks of the inferred heat flux profiles, these measurements allow an assessment of the local sheath heat flux transmission coefficient (γ). We report measurements of the sheath heat flux transmission coefficient at multiple locations in the scrape-off-layer (SOL) during an EDA H-mode discharge. The LP data are consistent with the presence of a heat flux “tail” measured by the IR system, extending into the far SOL. Measured γ are found to deviate from theoretical values by up to a factor of 2, depending on position in the SOL, pointing to some as yet unmeasured physical process. Calorimeter-inferred energy deposition profiles are found to be within 20% of the IR-inferred values across the footprint.